If you spend more than a little time in your Web browser, you probably take notice of upgrades. The hottest Web browser, Mozilla Firefox (www.mozilla.com), has again created considerable buzz, and this time only for an incremental upgrade, from version 3.0 to 3.5, released June 30.

Firefox, a product of Mozilla Corp., has a storied heritage and is a favorite of many Internet users as much for this as for its speed and features.

Firefox can trace its lineage back to the first widely used graphical World Wide Web browser, Mosaic, released in 1992 and responsible more than anything else for the explosion of the Internet into popular consciousness in the mid-1990s. Development of Mosaic stopped in 1997, but anyone curious can still give it a spin by downloading it from its original developer, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign (ftp://ftp.ncsa.uiuc.edu/Web/Mosaic).

In 1994 Mosaic spawned Netscape Navigator, whose popularity prodded Microsoft to release Internet Explorer a year later. The browser war of the late 1990s caused Microsoft antitrust headaches when the U.S. District Court for the District of Columbia ruled in 1999 that Microsoft had illegally thwarted competition by among other things providing Internet Explorer with copies of Windows installed on new PCs and preventing PC makers from including competing browsers. Before the popularity of broadband, downloading another browser for many users wasn’t worth the time.

In fairness, Internet Explorer had surpassed Netscape in quality by then, with the latter having grown fat and slow. Just after it started declining, Netscape was purchased in 1998 by America Online, which slowed and eventually stopped development of it. Internet Explorer enjoyed its peak popularity in 2002 and 2003, with an estimated 95 percent of the market.

Firefox was released in 2004 to try to stop the Internet Explorer juggernaut, and it has gradually siphoned off market share. As of June 2008, it had 22.5 percent of the market, according to Web metrics firm Net Applications (www.netapplications.com), despite the fact that Internet Explorer still comes bundled with Windows and to use Firefox you typically still have to download it.

The newest release of Firefox is worth the download, and millions of people are doing just that. During its first 24 hours of availability, version 3.5 was downloaded nearly 5 million times. The program’s current download status can be tracked at Worldwide Firefox Downloads (downloadstats.mozilla.com).

Compared to its predecessor, Firefox is faster and more stable. Firefox 3.5 also adds some interesting new features, particularly if you’re privacy conscious. By invoking private browsing mode, you prevent Firefox from recording what you do with it — history, cookies, usernames, and passwords. Another new privacy feature lets you direct Firefox to delete such information after the fact about individual sites (though it’s easier to do this for all sites).

Firefox comes in versions not only for Microsoft Windows but also for Mac OS X and Linux, in English as well as several dozen other languages. Firefox and Internet Explorer, though dominating the market, aren’t the only browsers in town, with Apple Safari (www.apple.com/safari), Google Chrome (www.google.com/chrome), and Opera (www.opera.com) rounding out the main competition.

Firefox, like many Web browsers, is free. Mozilla Corp. earns most of its revenue from search engine royalties, particularly through Google. In the upper right hand corner of Firefox, you can change the default search engine from Google to anything you want, including Yahoo or, moving further afield, Wikipedia for an encyclopedia search or Amazon.com for a shopping search.

With any Web browser, enhancing its features with programs created by other companies is fun and sometimes necessary. Such programs are called add-ons, with plug-ins extensions, and themes being subcategories.

Mozilla’s site indicates that more than 5,000 add-ons are available for Firefox. You can access a list of recommended add-ons from within Firefox by pulling down the Tools menu, selecting Add-ons, and then clicking on Get Add-ons.

The most common plug-ins include Acrobat (get.adobe.com/reader) for reading Web pages designed as print pages, Flash (get.adobe.com/flashplayer) for viewing animations and video, Java (www.java.com/getjava) for interactivity, and Windows Media Player (www.microsoft.com/windows/windowsmedia) for music and video.

My favorite extension is Forecastfox (addons.mozilla.org/firefox/398), which puts up-to-date weather information for your area in Firefox’s status bar, including the current weather and precipitation as well as forecasts for the next two days.

Computer magazines and Web sites also regularly publish lists of recommended add-ons. If you try an add-on and don’t like it, you can simply uninstall or disable it from within Firefox.